Detecting certain diseases or conditions of the eye often requires comparing photographic representations of the eye's component parts. Typically, such photographs are taken over the course of years by different examiners using different devices. These inevitable variations often result in an unavailing comparison that frequently frustrates the diagnosis of blinding diseases, like glaucomatous optic neuropathy (“glaucoma”).
Glaucoma is a blinding disease associated with progressive impairment of optic nerve function. Diagnosis and management of the patient with, or at risk for, glaucoma is highly dependent on detecting contour changes in the optic nerve head. Currently, detecting such contour changes in the clinical environment requires careful visual inspection of sequential, magnified, stereoscopic optic nerve head photographic image pairs that have been acquired at distinct instances in time. These image pairs may be acquired by a standard fundus camera, scanning laser ophthalmoscope, slitlamp camera, or other device capable of imaging the eye or its component parts.
With glaucoma, an examiner uses a stereoscopic viewer to view various images of the optic nerve head. Contour changes are observable as apparent differences in depth between the images. Discerning such changes through visual inspection, however, is difficult for patient management and clinical research trials. In particular, the examiner cannot superimpose the stereoscopic images, but must attempt to do so in his mind. This endeavor is made more difficult by the requisite time interval between captured images (often years), which invariably results in different magnification, rotation, position, and warp among the different captured images. As a result, detection of glaucoma using a stereoscopic viewer is notoriously difficult. Similarly, change detection based on images of the eye or its component parts is difficult whether derived from fundus photography, angiography, slitlamp photography, or other sources.
As a result of the difficulty in evaluating stereoscopic images of the optic nerve head, many clinicians compare visually only the information available on monocular photographs of the optic nerve head. In other words, they compare monocular photographic images of the optic nerve head taken over time, typically over the course of years, seeking to identify changes such as advancing atrophy or altered position of the retinal vessels. The unavoidable variations (e.g., exposure, orientation, and magnification) in photographs obtained over the course of years limit the diagnostic sensitivity of this manual approach to glaucoma diagnosis.
Goldmann and Lotmar, in extending a technique they termed “stereo-chronoscopy,” proposed that pairing monocular sequentially obtained photographic images of the optic disc could improve glaucoma diagnosis and management. Goldmann H. and Lotmar W., Rapid Detection of Changes in the Optic Disc: Stereo-chronoscopy, Albrecht v. Graefes Arch. klin. exp. Ophthal. 202: 87-99 (1977); Goldmann H. and Lotmar W., Rapid Detection of Changes in the Optic Disc: Stereo-chronoscopy. II. Evaluation Technique, Influence of Some Physiologic Factors, and Follow-Up of a Case of Choked Disc., Albrecht v. Graefes Arch. klin. exp. Ophthal. 205: 263-277 (1978). In this technique, monocular photographic views of the optic nerve head taken at different times were viewed simultaneously as a pseudo-stereo pair in a stereoscope. Optic disc change over time theoretically would appear as a pseudo-stereo effect in these paired images, while stable discs would appear flat to the observer. This method presupposed the capability of clinical photographic and optical methods to obtain images that could later be aligned visually. Because it proved impractical to take optic nerve photographs initially with the alignment precision demanded by the technique, stereo-chronoscopy never progressed beyond the initial pilot development stages.
A number of early studies attempted to improve on stereo-chronoscopy by employing various alternative means to achieve image superposition. For example, Heiji A. and Bengtsson B., Diagnosis of Early Glaucoma with Flicker Comparisons of Serial Disk Photographs, Invest. Ophthalmol and Vis. Science 30: 2376-2384 (1984) developed a crude form of flicker comparison by alternating between images from a projector device. This technique also allowed for manual alignment of the projectors to correct for translational and rotational misalignment between the sequential images. The methods of these authors were complex and time-consuming, but they did conclude that flicker analysis could be useful clinically.
Algazi R. V., Keltner J. L., and Johnson C. A, Computer Analysis of the Optic Cup in Glaucoma, Invest. Ophthalmol Vis. Science 26: 1759-1770 (1985) also described an early approach based on registration of sequentially acquired images with a reference image. The user could control rotation, translation and scale in order to bring these images into alignment. Sequential monocular display then facilitated change detection. The authors noted that the procedure was very time consuming when compared with standard techniques and that results were not reproducible.
Nagin P., Schwartz B., and Reynolds G., Measurement of Fluorescein Angiograms of Optic Disk and Retina Using Computerized Image Analysis 92: 547-552, Ophthalmology (1985) registered angiographic optic disc images to permit analysis of vascular filling in these image sequences. Vascular crossing points were identified automatically, and correspondence between these points allowed for determination of an average translational and rotational displacement vector describing the image transformation. However, this technique was not formally validated. Moreover, accurate fundus image change detection requires image registration beyond that permitted by rigid body transformations (i.e., rotation and translation).
Other methods have been developed that typically using complex optical instruments to obtain clinical images and provide quantitative measures of optic nerve head topography. However, because of their complexity, questionable accuracy, and uncertain diagnostic superiority, none of these newer methods has achieved widespread use or acceptance. Moreover, these complex methods often are unable to use archived photographic images of the patient's optic nerve head, which are important in detecting the progressive changes attributable to glaucoma. Thus, subjective assessment of standard clinical examination and standard fundus photographic images remains the primary method for diagnosing glaucoma.